Micro-machined optical switching devices for use in optical switching applications, often referred to as Micro Electromechanical Systems (MEMS) or Micro Opto Electromechanical Systems (MOEMS) and referred to hereinafter as a MOEMS, typically contain optical mirrors that are controllable electronically. The optical mirrors are typically micro-machined from a silicon wafer and coated with various materials to produce a reflective mirror surface. The mirror structure is typically bonded onto a substrate, specifically within a pre-formed cavity in the substrate. An optical transparent window (referred to hereinafter for convenience as a “cover”) is typically bonded onto the substrate across the cavity. The cover is typically a glass material, such as borosilicate glass or fused silica. The cover allows light to pass to and from the optical mirrors and protects the extremely fragile mirrors.
The substrate or silicon wafer typically includes electrode pads that are used to control the position of the optical mirrors, and also includes various electrical contacts. The optical mirrors must be positioned a precise distance above the electrode pads because they are controlled through electrostatic forces, and the voltage required to position a mirror depends on, among other things, the distance of the mirror from the electrode pads. Variations in the distance between the mirrors and the electrode pads make it difficult to control the position of the mirrors.
Reflections produced by the MOEMS cover surfaces can impact the optical switching performance of the MOEMS. Therefore, an antireflective (AR) coating is typically placed on one or both MOEMS cover surfaces to reduce reflections.
Electrostatic charge buildup on the MOEMS cover surfaces can degrade the positional accuracy and stability of the MOEMS mirrors. One solution is to make the cover (or its surfaces) conductive so the cover can be grounded, specifically by applying an electrically conductive film to the cover surfaces. Unfortunately, most conductive materials are opaque. Certain conductive inorganic oxides, often based on ITO (indium-tin oxide), have been used for similar applications in which electrically conductive surfaces are required on optically transparent windows (e.g., solar cells, photodetectors, and cathode ray tube (CRT) surfaces, to name but a few), although such conductive inorganic oxides typically provide insufficient transparency in the near infrared region at which the MOEMS typically operate, particularly at wavelengths around 1.3 microns and 1.5 microns (1.31μ and 1.5μ).